Its sorta like "seawater will rust steel eventually" vs we make all our ships out of steel. The ancient Boeing report is quoted at length as gospel but the rea

Its sorta like "seawater will rust steel eventually" vs we make all our ships out of steel. The ancient Boeing report is quoted at length as gospel but the reasoning is not explained. Here's some weird facts: 1) Most metals are not happy with NTO because any moisture turns NTO into concentrated nitric acid and nitric eats lots of metals. 2) Some industrial nitric acid plant components are made of very expensive titanium because its strong and essentially nitric acid corrosion proof. This makes Ti sound fantastic for NTO... however... 3) It is true that titanium is one of the few metals you can dunk in seawater where stress-corrosion does NOT occur. Which makes people even more hyper optimistic about Ti for NTO tanks as per above... however... 4) Unfortunately titanium DOES stress corrode in NTO. Stress corrosion is where essentially microcracks that would normally heal in other metals instead get a higher volume "rust" oxidation in the crack slowly breaking the metal apart after thousands or millions of cycles even if an unstressed part would never wear. Think of a perfectly painted garage that would never have wood rot due the the paint protecting the wood, but if you bend the wood back and forth tiny cracks will form in the paint and the water will soak thru the cracks into the wood and the garage wood will rot eventually even if the paint was theoretically perfect under unstressed conditions. Its actually a little more complicated than this. So from an engineering standpoint if you build something under millions of cycles of repetitive stress like the shaft of a circulation pump in an industrial production NTO tank out of titanium, you're doomed to have it shear in a "sooner than economically viable" amount of time like a couple months of continuous use maybe. On the other hand if you build a valve that will only exist for a little while and only be under a couple stress cycles in its lifetime and only possibly operate once for a few seconds, thats very likely OK. There is also a systems issue where if a life-critical failure occurs upstream resulting in everyone being dead already, it doesn't matter if downstream a system that should never be contaminated fails and burns. Or rephrased if enough stuff upstream broke such that the helium system literally caught fire, the crew are already dead men and the ships already lost so it doesn't really matter much anymore if the helium system burns or does not burn at that point. I mean its technically interesting but if you're gonna lose the vehicle and pax it doesn't matter really in the long run. Kinda like the old line about how we could build jet liners to the same standards as black boxes, but the external G forces of the crash would kill the passengers inside the giant black box so realistically why bother. If something upstream fails badly enough that the helium system is compromised, they're already dead anyway so engineering a theoretically indestructible helium system is a waste of time if the mission, ship, and passengers are already lost, so may as well save the weight and labor.